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Cycles of concentration is how many times more concentrated your tower water is than your make-up water — in practice, tower conductivity divided by make-up conductivity. Because evaporation removes pure water and leaves the minerals behind, the only way to stop the tower concentrating forever is to throw some water away, which is blowdown. Blowdown equals evaporation divided by (cycles minus one). That formula is why the first few cycles matter enormously and the later ones barely register: going from 3 to 6 cycles cuts blowdown by 60% and total make-up by 20%, while going from 6 to 9 cuts blowdown by only a further 38% of an already much smaller number. The ceiling is set by scale and corrosion chemistry, not by arithmetic.
A cooling tower rejects heat by evaporating water. Evaporation takes the H₂O and leaves everything dissolved in it behind. So the minerals in your make-up water — calcium, alkalinity, chloride, silica — steadily concentrate in the circulating water. Left alone, they would concentrate without limit until they precipitated onto your heat exchanger. The only defence is to deliberately dump some of the concentrated water and replace it with fresh: blowdown. Cycles of concentration is the ratio you settle at.
You will also see it defined against total dissolved solids. Conductivity is the practical field measurement and tracks TDS closely enough, which is why the controller on your tower is measuring conductivity.
Three equations do all the work. Evaporation is fixed by your heat load — you cannot reduce it, it is the entire point of the tower:
Cycles = tower conductivity ÷ make-up conductivity
Blowdown = Evaporation ÷ (Cycles − 1)
Make-up = Evaporation + Blowdown
The second equation is the whole story. Look at what the (Cycles − 1) denominator does:
| Cycles | Blowdown as a multiple of evaporation | Change from previous row |
|---|---|---|
| 2 | 1.00 × E | — |
| 3 | 0.50 × E | halved |
| 4 | 0.33 × E | −33% |
| 6 | 0.20 × E | −40% from 4 cycles |
| 8 | 0.14 × E | −29% from 6 cycles |
| 10 | 0.11 × E | −22% from 8 cycles |
This is a curve of diminishing returns, and it is steep. The move from 2 to 4 cycles saves more water than the move from 4 to infinity. If you take one thing from this article: the money is in escaping low cycles, not in chasing high ones. Anyone selling you a programme on the promise of 10 cycles is selling you the flattest part of the curve — and, as below, the riskiest.
Enter your system. Everything updates live. Nothing is sent anywhere.
| At 3 cycles | At 6 cycles | |
|---|---|---|
| Tower conductivity | — µS/cm | — µS/cm |
| Blowdown | — m³/h | — m³/h |
| Make-up | — m³/h | — m³/h |
| Make-up per year | — m³ | — m³ |
| Water cost per year | — | — |
Evaporation is estimated as circulation × range × 0.0017, the standard approximation behind the old “1% of circulation per 10°F of range” rule. Drift loss is ignored — on a tower with working drift eliminators it is small enough not to change the conclusion, and leaving it out makes the saving estimate slightly conservative. Default water cost is the WSD trade tariff of HK$4.58/m³; your real figure should include sewage charge, so raise it accordingly.
WhatsApp these numbers to GotechIf the arithmetic says higher is better, why does nobody run at 15 cycles? Because concentrating the water is exactly the thing that causes scale and corrosion. Every cycle you add multiplies the mineral content, and four constraints arrive:
So the real question is never “how high can cycles go”. It is “how high can cycles go on this water, with this treatment, on this metallurgy” — and that is a chemistry answer, not a spreadsheet answer.
In our experience it is almost never a deliberate decision. It is usually one of these:
Every one of those is cheap to find. Measure make-up conductivity and tower conductivity on the same visit and divide. If the answer is under 3, there is money on the floor.
Water is the obvious saving, and on the trade tariff it is a real but modest number. Three larger effects usually ride along with it:
And the counterweight, stated plainly: if raising cycles causes scale, none of these savings survive contact with the energy bill. A thin film of calcium carbonate on a condenser costs more in compressor energy than the entire water saving is worth. Cycles optimisation that is not paired with an inhibitor programme capable of holding the new chemistry is not optimisation. It is a bet.
There is no universal number, because the ceiling is set by your make-up water chemistry, your metallurgy and your treatment programme, not by a rule of thumb. Many towers run at 2 to 4 cycles and 6 or more is often achievable. What matters is which parameter caps first — commonly calcium carbonate scale potential, chloride, or the EMSD Code's indicative conductivity criterion of 1,500 microsiemens per centimetre.
Divide the conductivity of the circulating tower water by the conductivity of the make-up water. Both readings should be taken on the same visit with the same calibrated instrument. Conductivity is used as a practical proxy for total dissolved solids.
Blowdown equals evaporation divided by cycles minus one, so the savings are front-loaded. Going from 3 to 6 cycles cuts blowdown by 60% and total make-up by about 20%. Going from 6 to 9 saves far less in absolute terms. The largest savings come from escaping low cycles, not from chasing high ones.
Yes. Concentrating the water is what causes scale and corrosion. Higher cycles multiply calcium and alkalinity, which drives calcium carbonate deposition on heat transfer surfaces, and multiply chloride, which attacks passive films and drives pitting. Cycles should only be raised alongside an inhibitor programme capable of holding the resulting chemistry.
Blowdown is the deliberate dumping of part of the concentrated circulating water so it can be replaced with fresh make-up. Evaporation removes pure water and leaves the dissolved minerals behind, so without blowdown the tower would concentrate without limit until minerals precipitated onto the heat exchanger. Blowdown equals evaporation divided by (cycles minus one), and make-up equals evaporation plus blowdown.
Most sites do not. We will measure both conductivities, check the controller is really controlling, and tell you what your water is capable of.